The con-elation between superoxide radical production and hepatic damage under cholestasis induced by common bile duct (CBD) ligation was studied in rats. The amount of superoxide radical production in the cholestatic liver was significantly increased after CBD ligation compared to a sham operation control group. Serum and cytosolic xanthine oxidase (XO) activities were significantly increased after operation. Serum alanine aminotransferase (ALT), aspartate aminotransferase (ASS, and alkaline phosphatase (ALP) activities, and the bilirubin level of the cholestatic liver in the subject rats increased after CBD ligation. Severe morphological changes were also noted after CBD legation. These parameters, which are known to reflect hepatic function in the serum, and the XO activity were significantly correlated with the amount of superoxide radical production. According to these results an increased XO activity includes an increased conversion from xanthine dehydrogenase in the liver, resulting in an increase in oxygen free radical production. These oxygen radicals may play a role in the aggravation of inflammation and necrosis in the liver under cholestasis induced by CBD legation.